The present invention relates to integrating analog-to-digital converter (ADC), particularly for digital systems requiring a fast and accurate high resolution conversion of an analog input voltage or current into a corresponding digital output code.
Two groups of ADCs can be distinguished: indirect and direct ADCs. Integrating type ADCs operate by indirect conversion method as the input signal is converted into a ramp signal which is applied to a comparator. A time period is measured by counting clock pulses. Several variations such as single slope, multiple slope and charge balancing ADCs exist on the basic principle. For example in the single slope ADC ramp voltage of an integrator is rising from ground potential and is compared with the ADC input voltage by a comparator. If the comparator switches, a counter counting clock pulses is stopped, wherein the result of the conversion is available at its output. In the dual slope ADC the conversion begins when the input signal is applied to the integrator for a fixed period of time determined by counting of a counter. Subsequently, the direction of the ramp signal is reversed. A reference voltage of an opposite polarity is applied to the integrator until the ramp voltage is back to zero. The counting is stopped when the zero crossing is detected by a comparator. The counter output represents the ADC output code.
The direct ADCs immediately digitize the input signal. Flash, multiple flash and successive approximation ADCs fall into this group. For example, in the dual flash ADC the input voltage is converted into most significant bits (MSBs) which are further converted by a digital-to-analog converter (DAC) into voltage. This voltage is subtracted from the input voltage, the result is amplified and further converted into least significant bits (LSBs). The MSBs and LSBs represent the ADC output code.
The integrating ADCs have a very high accuracy, large noise rejection, ability of an effective suppressing periodic interference signals, extremely small nonlinearities, inherent monotonicity and virtually no missing codes. They are surpassed by the direct ADCs only in a high maximum conversion speed.